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Peter Anvin" Subject: [PATCH v2 2/2] x86/smp: Use atomic_try_cmpxchg in native_stop_other_cpus() Date: Thu, 23 Nov 2023 21:34:23 +0100 Message-ID: <20231123203605.3474745-2-ubizjak@gmail.com> X-Mailer: git-send-email 2.42.0 In-Reply-To: <20231123203605.3474745-1-ubizjak@gmail.com> References: <20231123203605.3474745-1-ubizjak@gmail.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Use atomic_try_cmpxchg() instead of atomic_cmpxchg(*ptr, old, new) == old. X86 CMPXCHG instruction returns success in ZF flag, so this change saves a compare after CMPXCHG. Tested by building a native Fedora-38 kernel and rebooting a 12-way SMP system using "shutdown -r" command some 100 times. No functional change intended. Cc: Dave Hansen Cc: Andy Lutomirski Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: Ingo Molnar Cc: Borislav Petkov Cc: "H. Peter Anvin" Signed-off-by: Uros Bizjak --- v2: Split from v1 patch. Simplify commit entry and state how the patch was tested. --- arch/x86/kernel/smp.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c index 97f6f0cbb43a..2908e063d7d8 100644 --- a/arch/x86/kernel/smp.c +++ b/arch/x86/kernel/smp.c @@ -148,15 +148,16 @@ static int register_stop_handler(void) static void native_stop_other_cpus(int wait) { - unsigned int this_cpu; + unsigned int old_cpu, this_cpu; unsigned long flags, timeout; if (reboot_force) return; /* Only proceed if this is the first CPU to reach this code */ + old_cpu = -1; this_cpu = smp_processor_id(); - if (atomic_cmpxchg(&stopping_cpu, -1, this_cpu) != -1) + if (!atomic_try_cmpxchg(&stopping_cpu, &old_cpu, this_cpu)) return; /* For kexec, ensure that offline CPUs are out of MWAIT and in HLT */ -- 2.42.0